1. Field of the Invention
The invention relates to a printed circuit board and a heat sink and, more particularly, to a printed circuit board supplied with large current and a heat sink capable of dissipating heat and conducting electricity.
2. Description of the Prior Art
When a high-power chip and an electronic component with high heating value, such as field effect transistor (FET), diode, triode, and so on, are disposed on a printed circuit board, heat dissipation and large current conduction for the chip and electronic component during operation are usually achieved by connecting a copper sheet or heat sink. In other words, the copper sheet with high heat dissipation and high electricity conduction can be used to achieve heat dissipation and current conduction. For large current, the copper sheet or heat sink has to be made of copper or brass with large thickness.
In general, the copper sheet or heat sink is soldered on the printed circuit board by soldering tin. Specifically, a soldering portion of the copper sheet or heat sink has to be immersed into a through opening of the printed circuit board. Then, the melted lead-free tin, which is provided by a lead-free tin soldering pot, is drawn upwardly along the soldering portion by capillarity. After solidifying the melted lead-free tin, the copper sheet or heat sink has been fixed in the through opening of the printed circuit board.
At the present time, the soldering process has been gradually changed from lead tin soldering process to lead-free tin soldering process, so as to satisfy restriction of the use of certain hazardous substance in electrical and electronic equipment (RoHS) enacted by European union. Since the lead-free tin has high melting point, the soldering process has to be proceeded in a soldering pot with high temperature so as to melt the lead-free tin, such that the soldering portion of the cooper sheet or heat sink can guide the melted lead-free tin upwardly. However, it is very easy and fast for the copper sheet or heat sink to dissipate heat from the lead-free tin due to high heat dissipation of the copper sheet or heat sink itself. Consequently, the melted lead-free tin will be solidified quickly and the cooper sheet or heat sink with large thickness will be soldered onto the printed circuit board incompletely in lead-free tin soldering process. To solve the aforesaid problems, the prior art usually forms a hole over the soldering portion to reduce heat dissipation of the cooper sheet or heat sink, so as to prevent the lead-free tin from solidifying in a short time. Although heat dissipation still can be maintained, the cooper sheet or heat sink cannot be adapted to large current due to insufficient cross section where the current is passing.
Therefore, it is necessary to provide a heat sink capable of being soldered well and conducting large current and a printed circuit board utilizing the heat sink.